This invention is related to manufacturing of Al-CNTs composites, in particular, a new technology of in situ synthesizing the Al-CNTs materials for structural and conductor applications.
Carbon nanotubes (CNTs), first discovered in 1991, are now well known to possess extremely high strength, elastic modulus, electrical and thermal conductivity, all while being of low weight construction. They are shaped as elongated hollow cylinders with honeycomb-like nanostructured graphene walls (i.e., that are no more than one atom in thickness). Despite these advantages, the development of metal matrix composites based on CNTs has not been forthcoming, due at least in part to the difficulty of dispersing the CNTs in a manner analogous to polymer- and ceramic-based CNT composites. In the cases of polymer and ceramic matrix composites, some reports have demonstrated that agglomerated CNTs with a high cohesive force could be broken up and homogeneously mixed into a matrix by utilizing their electro-kinetic potential; from this, the resultant composites can exhibit the desired structural performance. Nevertheless, these approaches are not effective for CNT dispersion in metals because of the low controllability of the zeta potential of metal particles and the large density difference between the metal and CNTs. Mechanical mixing and hot extrusion has been proposed as a way to overcome these issues; however, the mechanical properties of resulting the Al-CNTs composites (an ultimate tensile strength of 84 MPa with a 5% by volume addition of CNTs), are not significantly improved relative to pure aluminum (discussed below). Another approach tried to mix CNTs with aluminum powders by highly efficient mixing using a tubular shaker-mixer and planetary mill; the difficulty of this approach was that the CNTs weren't adequately dispersed, causing electrical performance to suffer.
One form of electric induction motor involves a rotating armature (rotor) surrounded by a coil-wound stationary field (stator). When electric current is passed through the stator windings, a part of the stator known as the pole (which may be made up of a magnetically permeable material, such as iron) around which the windings are wrapped becomes magnetically energized, which in turn imparts an electromagnetic force to the rotor, causing it to rotate. In motive applications, a shaft attached to the rotor can be used to provide propulsive force to a vehicle through the turning of one or more linked wheels. Such a motor could be especially useful in vehicles that rely either entirely on electric power, or as part of a hybrid system, where the electric motor and an internal combustion engine (such as conventional gasoline or diesel variant) cooperate with one another to produce the desired motive force.
A “squirrel-cage” rotor is a common example of an electric induction motor, and derives its name from its cage-like cylindrical shape, where numerous metal rotor bars or rods extend longitudinally and are spaced around the cylindrical periphery of a central axis of rotation. The bars are held in a fixed relationship to one another by metal end rings so that adjacent bars and connected end rings form a cage-like structure with numerous coil-like electrically continuous loops. Due to the proximity of the rotor to the stator, changes in the magnetic field produced in the stator induce current in the highly conductive loops formed by the bars and end rings. This current turns the rotor into an electromagnet that can spin in an attempt to align itself with the magnetic field produced in the stator. To increase the magnetic intensity of the rotor, a series of laminated plates (typically made from a material that has a lower magnetic resistance (i.e., more magnetically permeable) than air, such as iron) are mounted to the shaft or related mandrel such that they occupy the substantial entirety of the space between the shaft and the cage formed by the bars and end rings. Moreover, a low electrical conductivity material (for example, a coating) could be used to minimize electrical contact between them. The cooperation of the laminated stack of plates with the current flowing through the loops of the cage help to strengthen the magnetic field generated by the loops of the rotor, and leads to higher levels of torque generated in the attached shaft. To keep the torque generated at a relatively constant level, the bars making up the cage may be skewed to define a slightly helical pattern rather than one that is strictly longitudinal.
In one form, the bars (also referred to herein as conductor bars) are made from a highly-conductive material, such as pure aluminum. While such material has excellent electrical conductivity, its mechanical properties (as alluded to above) tend to be limited, thereby hampering its ability to meet durability requirements, especially in the harsh environments associated with automotive applications. This problem is exacerbated by the bars' inherently high aspect ratio and attendant increased susceptibility to defects in casting and other conventional fabrication techniques. While alloying elements can be added to the aluminum to improve its strength or related mechanical properties, electrical performance tends to become compromised through the lower conductivity.
Numerous fabrication techniques have been used to produce the bars and end plates (or end rings) of an induction motor. For example, the bars and end plates or rings may be created as separately-formed structures that are then joined together through welding, fastening, adhesives or the like. As with the connection between the plates of the laminate stack, a non-conducting adhesive may be used to secure the bars to similarly sized and shaped slots formed in the laminated plates. Another fabrication technique for longitudinal metal bars involves casting, where molten aluminum may be poured directly into the slots once the laminate plate structure has been assembled. Casting of a squirrel-cage rotor is advantageous relative to assembling it from separate parts, as it reduces the cost and manufacturing variances associated with assembled components, although it is hard to make defect-free conductor bars with traditional casting techniques. Moreover, neither casting nor joining are able to produce bars or their resulting cages that simultaneously satisfy the stringent electrical and structural needs of an induction motor that is based on such bars and cages.